The present invention relates to an overglaze color for pottery, and in particular to an overglaze color for pottery not containing harmful substances such as lead, cadmium, etc.
In the normal procedure for manufacturing pottery, a raw material is processed to unglazed pottery at 600.degree.-1000.degree. C. and underglaze decoration is then drawn with pigments such as cobalt, manganese, etc. Then, glaze (frit) is coated on the decoration and the pottery is fired at about 1300.degree. C. Thereafter, the overglaze colors (frit) containing pigments are applied as a coating, and the pottery is fired again at about 800.degree. C. This firing temperature is very important for the maintenance of the melting property of the overglaze frit, for the prevention of devitrification and cracking as well as for effective color development of pigments.
Basic conditions of overglaze colors for pottery are: (1) at the firing temperature of about 800.degree. C. where the colors and the pigments develop color to the utmost extent, the thermal expansion coefficient must not differ from that of the glaze fired at a high temperature of 1300.degree. C.; (2) the frit component must have no devitrification property and must develop color together with pigment when the latter is added.
To prepare the conventional type overglaze colors for pottery, alkali metal oxide and boron oxide are added to silicon oxide and alumina derived from silica rock, kaolin, etc. in order to improve the melting property, and zirconium oxide and zinc oxide are added to increase chemical durability. To the frit thus prepared, pigment is added. Because acid resistance is reduced when alkali metal oxides are used in a large quantity, lead oxide with a high melting property and good affinity with pigment is used as an essential component.
Further, cadmium oxide is added to increase the color developing property of the pigment. Thus, the conventional type overglaze colors for pottery contain heavy metals such as lead, cadmium, etc. in amounts as high as 70 wt % in some cases. Therefore, heavy metals such as lead, cadmium, etc. are eluted when pottery is used, and it is pointed out that such components cause harmful effects when the pottery is used as tableware such as dishes.
To solve the problems, various attempts and studies have been made to reduce the elution of heavy metals such as lead, cadmium, etc. and to increase acid resistance by improving firing conditions. For example, an attempt has been made to increase the resistance to acid by adding a trace quantity of zirconium oxide to the lead silicate type frit (Journal of Japanese Ceramic Industry Association; 83 (2), 81; (1975)). Or, a special kiln for underglaze decoration is used, or a special charging procedure into the kiln is invented. Further, attempts have been made to equalize the intra-kiln temperature, or to increase temperature gradually within the temperature range of about 400.degree.-500.degree. C., to maintain the temperature at more than 760.degree. C. for 30-60 minutes, or to promote degassing in the kiln.
However, because the conventional type overglaze colors basically contain lead and cadmium in their chemical composition, it is impossible to eliminate the elution through the addition of an acid-resistant component or through the improvement of firing conditions.
In the safety standards for food additives in accordance with the provisions of the Food Hygiene Law, Article 10 Paragraph 1, it is provided that the elution quantity after immersion in 4% acetic acid aqueous solution for 24 hours must be less than 20 ppm for pottery to be used for liquid, less than 7 ppm for pottery not to be used for liquid, and 2.0 ppm for a container to keep food. In conventional type overglaze colors for pottery, harmful metal exceeding the safety standard values is often eluted even with a slight change in firing conditions.
An object of this invention is to provide overglaze colors for pottery, which contain no harmful metal components such as lead, cadmium, etc. but which have the same properties as conventional lead-containing overglaze colors for pottery, while maintaining an acid resistance property. It is also to provide a method for manufacturing such overglaze colors.